1. Field of the Invention
The present invention relates to a rotational speed differential responsive type control coupling, particularly of the kind having a variable transfer torque control means, i.e., a means for variably controlling a transfer torque for a given rotational speed differential.
2. Description of the Prior Art
An example of such a control coupling is disclosed in European Patent Publication No. 0,398,124. In such a control coupling, a stepping motor is utilized for variably controlling the axial position of a valve spool for thereby controlling the torque transfer characteristic through control of the size of the variable orifice.
However, if a control circuit of such a stepping motor malfunctions, the valve spool is moved under the pressure of the hydraulic fluid in the accumulator chamber until it abuts upon the stopper and fully opens the orifice. By this, the control coupling is disabled to perform a torque distribution function or a differential slip limiting function.